udp6: fix jumbogram reception

Since commit 67a51780aebb ("ipv6: udp: leverage scratch area
helpers") udp6_recvmsg() read the skb len from the scratch area,
to avoid a cache miss.
But the UDP6 rx path support RFC 2675 UDPv6 jumbograms, and their
length exceeds the 16 bits available in the scratch area. As a side
effect the length returned by recvmsg() is:
<ingress datagram len> % (1<<16)

This commit addresses the issue allocating one more bit in the
IP6CB flags field and setting it for incoming jumbograms.
Such field is still in the first cacheline, so at recvmsg()
time we can check it and fallback to access skb->len if
required, without a measurable overhead.

Fixes: 67a51780aebb ("ipv6: udp: leverage scratch area helpers")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

2 files changed, 11 insertions, 1 deletions

diff --git a/net/ipv6/exthdrs.c b/net/ipv6/exthdrs.c
index 4996d734f1d2..3cec529c6113 100644
--- a/net/ipv6/exthdrs.c
+++ b/net/ipv6/exthdrs.c
@@ -756,6 +756,7 @@ static bool ipv6_hop_jumbo(struct sk_buff *skb, int optoff)
 	if (pskb_trim_rcsum(skb, pkt_len + sizeof(struct ipv6hdr)))
 		goto drop;
 
+	IP6CB(skb)->flags |= IP6SKB_JUMBOGRAM;
 	return true;
 
 drop:
diff --git a/net/ipv6/udp.c b/net/ipv6/udp.c
index 98fe4560e24c..578142b7ca3e 100644
--- a/net/ipv6/udp.c
+++ b/net/ipv6/udp.c
@@ -328,6 +328,15 @@ struct sock *udp6_lib_lookup(struct net *net, const struct in6_addr *saddr, __be
 EXPORT_SYMBOL_GPL(udp6_lib_lookup);
 #endif
 
+/* do not use the scratch area len for jumbogram: their length execeeds the
+ * scratch area space; note that the IP6CB flags is still in the first
+ * cacheline, so checking for jumbograms is cheap
+ */
+static int udp6_skb_len(struct sk_buff *skb)
+{
+	return unlikely(inet6_is_jumbogram(skb)) ? skb->len : udp_skb_len(skb);
+}
+
 /*
  *	This should be easy, if there is something there we
  *	return it, otherwise we block.
@@ -358,7 +367,7 @@ try_again:
 	if (!skb)
 		return err;
 
-	ulen = udp_skb_len(skb);
+	ulen = udp6_skb_len(skb);
 	copied = len;
 	if (copied > ulen - off)
 		copied = ulen - off;